Slashdot videos: Now with more Slashdot!

View

Discuss

Share

We've improved Slashdot's video section; now you can view our video interviews, product close-ups and site visits with all the usual Slashdot options to comment, share, etc. No more walled garden! It's a work in progress -- we hope you'll check it out (Learn more about the recent updates).

mlimber writes "The New York Times Magazine has a lengthy article on dark matter and dark energy, discussing the past, present, and future. 'Astronomers now realize that dark matter probably involves matter that is nonbaryonic ["meaning that it doesn't consist of the protons and neutrons of 'normal' matter"]. And whatever it is that dark energy involves, we know it's not 'normal,' either. In that case, maybe this next round of evidence will have to be not only beyond anything we know but also beyond anything we know how to know.'"

Large black holes are located at the center of galaxies, and their mass can be determined by examining rotation curves, etc. They are not dark matter candidates. Primordial black holes are not massive enough. There is some possibility that dark matter could be non-luminous dust, but there are some limits placed on observations of the comsic microwave background, which would have had to travel over 13 billion light years through such dust without being significantly attenuated.

The 'size' of the universe is an ill-defined question. We can only observe what's in our past light cone, and it is *that* universe which suffers from a budget shortfall of matter/energy.

There is some possibility that dark matter could be non-luminous dust, but there are some limits placed on observations of the comsic microwave background, which would have had to travel over 13 billion light years through such dust without being significantly attenuated.

Galactic dark matter, which is required to explain the rotation curves of spiral galaxies, can be completely explained by baryonic dark matter, which would be at least partially dust.

Extra-galactic dark matter cannot be primarily baryonic. The baryon density of the universe is known from big bang nucleo-synthesis and the primordial H/He ratio, and is too small to account for extra-galactic dark matter. Therefore extra-galactic dark matter has no relation at all to galactic dark matter, as it cannot be made of the same stuff as galactic dark matter.

So there are at least two completely different, totally unrelated dark matter problems. One can and probably is solved by baryons. The other requires exotic particles or possibly exotic physics.

So there are at least two completely different, totally unrelated dark matter problems.

You're right that the universal baryon density doesn't specifically constrain galactic dark matter. But Occam's Razor suggests there is only one dark matter problem. Besides, you would have to explain why galaxies would have one type of dark matter while galaxy clusters have a completely different kind (and we know intra-cluster dark matter is non baryonic). It's much easier to explain the dark matter evidence at all s

It's true that there are multiple scales to the dark matter problem, and that our arguments for exotic dark matter apply on the extra-galactic scale. I don't think theorists seriously argue that baryons solve the galactic dark matter problem, however. The Bullet cluster result (Google for Sean Carroll's excellent piece on this) tells us that the dark matter in galaxy clusters can't be baryonic either (it interacts too weakly with ordinary matter). The numbers we have from various experiments add up best if even galaxies are dominated by dark matter halos.

Evidentally, some part of the universe simply has not yet been observed and recorded.

A (potentially gigantic) chunk of the universe cannot be observed and recorded. The initial expansion of the universe exceeded the speed of light, and therefore there is matter so far away from us that its light has not yet reached us.

As a simple thought experiment to convince you that the expansion was superluminal: according to the Big Bang theory, the universe exploded from a singularity. Well, what happens when you ha

Here [cosmicvariance.com] is an excellent article by Sean Carroll [preposterousuniverse.com] of the California institute of Technology that explains why all the suggestions of the parent post may not be correct.

Basically, what it says is that if two large clusters of galaxies went right through each other, and dark matter was really like the normal matter in the way the parent post suggests, we would get a different result from what would happen if dark matter was for real. Astronomers have discovered one such system and this provides conclusive evidence for the existence of dark matter.

The problem is that the Dark Matter seems to be moving at a much faster rate than expected. So now the Relativists are posing a new interaction that only works between Dark Matter;-). They don't concede defeat easily, those Relativists.

Or you can have an object that doesn't interact electromagnetically, which means it won't absorb most kinds of energy and won't radiate any convenient EM waves for us to detect.

Since we've only ever looked for matter electromagnetically, is it that surprising that now that we're starting to use new detection methods we're starting to see new things? Gravity wave detectors may see a very interesting universe.

Very large bodies don't behave according to Newton. Very small bodies behave according to the rules of quantum physics, so it's clear that one law doesn't regulate every case. Dark matter/energy are just a fudge factor because we can't explain what happens without them, but that doesn't prove that they exist. All that is proven is that we don't understand what is happening.

On the contrary, very large bodies are extremely well-approximated by Newton, as it is the slow-velocity, weak field limit of General Relativity. There is already good photographic evidence for dark matter in the form of colliding galaxies (do your Google work), and current observational evidence points pretty strongly towards dark energy in the form of a cosmological constant. While it's true we don't know what that means, it's not just a fudge factor.

I get increasingly annoyed by people who claim to use Occam's Razor to dissmiss the existance of a god, but are only too willing to eat up the lastest babble about dark matter (or string theory, etc, etc).

Thank goodness that there are people like Moffat that use reason and intellect to look at the world. I mean really - "there's all this stuff in the universe, but it's INVISIBLE man, so you know, you can't like see it or anything...." - dark matter doesn't come close

The colliding galaxies you are talking about is not really colliding galaxies, but colliding clusters of galaxies, this one is called the Bullet Cluster. A cluster of galaxies is a huge body of matter, which is not adequately explained by the only other competing postulate MOND. MOND requires some dark matter to exist for it to fit the cluster of galaxies, but it can do so currently by using hot and massive neutrinos, instead of unobserved weird dark matter particles.But the real problem with using dark mat

I thought this too for a long time, but it seems that the only evidence for dark matter isn't just galactic rotation curves. I'm having trouble finding it through Google, but while I was studying astrophysics last year we were shown an image of a gravitationally lensed quasar, but without any visible foreground stars. The lensing may have been caused by a clump of baryonic matter that just happened to be cold and not emitting much light, but it may also be dark matter. So unfortunately it's not quite as sim

I've only done an undergraduate degree majoring in physics, that guy's work is beyond me:p. I'm really glad that people are thinking about this stuff in ways other than just accepting the existance of dark matter and basing all their maths on that assumption. The "greatest blunder of Einstein's life" was when he assumed the universe was static.

Actually MOND makes it more easy for Dark Galaxies made up of hot and massive neutrinos to exist, because it makes gravity stronger. So yes, entire galaxies of hot and massive neutrinos are possible, and probably are present in most Clusters of galaxies. MOND shows that there is some matter missing from these Clusters. So the presence of Dark Matter is not fatal to MOND. The presence of Cold Dark Matter may be problematic, but still not fatal. The only thing that will be fatal to MOND will be if we find a b

Very large bodies don't behave according to Newton. Very small bodies behave according to the rules of quantum physics, so it's clear that one law doesn't regulate every case.

Don't forget that this "law" is simply an equation based on observable evidence. If it doesn't govern very large bodies, it simply means the equation is incomplete and missing one or more variables that start to matter at large scale.

It is not just a equation. It is a theory, which means that it contains a lot more features than a single equation. The equation is actually derived from the theory. The theory ie GR is very simple conceptually, that the speed of light is constant and mass curves the space it exists in. Everything follows from there. To change it will be very difficult. Now GR has 10 parameters of symmetry, we could go to higher symmetry eg 15, which is Conformal Symmetry and is observed for the other 3 forces. This will ca

Steven Weinberg the Nobel prizewinner and another physics faculty member used to travel in the same physics department lift, and they would exchange pleasantries. This faculty member had a very bright graduate student. Weinberg had not seen him around for some time, so he asked his colleague what had happened to the grad student. He told Weinberg "He tried to understand quantum mechanics". Both men sighed and then exited the lift at their respec

I understand that Quantum Physics is a set of equations which don't make much sense outside the realm of its mathematics, but it does make testable predictions.Now Dark Matter is a different beast. It is added to fit any observation. It does not allow you to make any prediction. It is simply a device to fit GR into observations. Normally this would not be a bad idea, except for the really unfortunate case that MOND does fit all galactic observations without needing any Dark Matter. This is really unfortunat

At this point, dark energy is really nothing more than a fudge factor. It's certainly nothing like the normal concept of energy. We don't even know if it's a cosmological constant or if it varies over time and space, let alone whether it's a property of spacetime or some form of particle. So far, I'm still unconvinced that it actually exists: it seems more likely to me that the current theories are simply slightly off in their formulas, and can be resolved without recourse to another of Occam's entities.

There is NO question that that expansion of the universe is accelerating. According to General Relativity, the ONLY way this can be happening is if the universe is dominated by a species with a negative pressure. If you're not happy with the name dark energy, call it 'quintessence', although this term has come to be applied to non-cosmological-constant dark energy, i.e., that provided by scalar fields in false vacuums, etc.

Sorry, but I think you completely ignored what the GP said and basically spouted a nonsensical stream of verbal diarrhoea which vaguely sounds like you know what the hell you are talking about when you actually don't. The GP was questioning the use of dark energy and dark matter as a kludge to make General Relativity work.

In my mind, we should not be looking for convenient stop gap solutions pulled out of thin air for this discrepancy between what is observed and General relativity but rather looking for

GR does not work at Galactic Levels, so there is no question of it working at the Cosmic levels.The real problem is MOND. If it did not exist then Dark Matter would be free to exist wherever it wanted. But with MOND the picture becomes more complex, now DM must fit MOND. It is quite easily provable that DM cannot fit MOND, just apply it to small cluster of stars at the outer edge of Milky Way which show Dark Matter. The problem is that for DM to fit Milky Way, it cannot be present in the Clusters. But some

So far, I'm still unconvinced that it actually exists: it seems more likely to me that the current theories are simply slightly off in their formulas, and can be resolved without recourse to another of Occam's entities.

I'm with you there - we either modify our understanding of gravity or we have to fill the universe with stuff and energy we can't see, detect, or comprehend, and if it's there it's completely unlike everything we can see. That could be true, but Occam's Razor doubts it.

Some questions that spring to mind:If the grand majority of the 'stuff' in existence around the universe is matter that would be somewhat alien to our range of experiences, could this have an effect on inter-galactic travel? Would what we think it is so far be matter we'd have to worry about hitting and being damaged by at very high speeds?

Is it dangerous? Would it be inert enough that it would be safe for life to come in physical contact with it?

Dark matter is not chemically interesting since, by definition, it doesn't interact with normal matter. Hence, it's unlikely to be 'useful' in any current fashion!

As far as whether it's dangerous -- if dark energy is a cosmological constant, it's a property of spacetime, and you are in a sense exposed to it right now. As for dark matter, again, it's something that would pass right through you, much like neutrinos.

If it doesn't interact by the electromagnetic force, it cannot affect anything chemically. If it doesn't interact by the strong force, it cannot cause nuclear reactions. Even if it interacts by the weak force, the effect would be equivalent to the neutrinos already coursing through us. To my understanding, it's an explanation for effects specifically by gravity, which we already are experiencing.

That same year, Michael Turner, the prominent University of Chicago theorist, delivered a paper in which he called this antigravitational force "dark energy."... "It really is very different from dark matter," Turner said. "It's more energylike."

As we are all aware, m(d) is the abbreviation for medical doctor (+|- the parentheses), so let's go ahead and substitute for physicists (and since the last parenthetical phrase says +|- the parentheses, lets get rid of the +|- and the parentheses), which would give us:Ed ==

My point being, don't you begin thinking we're some sorta odd artifact in the universe. It's the wrong way to think about it. Not to mention I believe all this "dark matter" and "dark energy" scientists are looking for is a result of improper equations which make us believ

Are you trying to say that all the great artists weren't weirdos, that the most innovative music doesn't come from marginalized cultures? It's the "abnormal" stuff that makes life and the universe interesting, the foam on the beer, the pepper on the pasta, yadda yadda.

I like thinking that the visible universe is just a kind of interesting foam clinging to the more mundane stuff.

I'm surprised that it took so long for Rumsfeld's name to crop up.Still, both as a fear of scientists and an artful dodge of politicians, The Unknowable is unlikely to leave us stranded at some cosmic stalemate. It just doesn't seem to be a feature of our universe.

And dark matter would be a strange place for it to happen. I'd be less surprised, in fact, if it turned out that ghosts were really some dark matter beings who could occasionally stumble into clouds of weirdness that permit them to interact with e

One thing I presently do not get is where the energy leaked from red shifted photons go.

Every photon is quantized. It is a particle, emitted when an atom change from an excited to a less excited state (basically, an electron change from an outer to an inner position). This photon get different levels of energy depending on how far they jump, and different frequencies of light correspond to the photons in that light having a higher energy level.

Dark matter is a mysterious attractive force operating on a scale of of hundreds of thousands of light years with about 24% of the universe's energy budget. Dark energy is a reulsive force operating a scale of billions of light years with 70% of universe's energy budget. Whether these are conventional particles, unknown particles, geometric effects, etc. it is not yet known.

"Since the invention of the telescope four centuries ago"
I didn't know telecopes were that old. Is this a typo, and didn't
they mean decades instead? If not, what did ancient telescopes do?

FooBarWidget, meet Galileo: Widely credited as the inventor of the modern
telescope, in 1609.

Though, as with all major developments in human history, some accounts have him
as merely improving on preexisting tech, whether copying the work of Lippershey
from 40 years before, or even the possibly MUCH older designs of the
ancient Persians.

Ancient telescopes were essentially spyglasses or binoculars, allowing one to see a great distance. And it's true that there is some evidence that arabs used such devices to study the stars even earlier. But it's generally regarded that the first telescopes in the modern sense appeared around 1600, and you've probably heard of Galileo, who made his own telescope in 1609 and then founded the modern science of astronomy via his observations.

No, it's not a typo. Although I would probably say even older. The old telescopes did much the same as new telescopes. They would allow a viewer to put his eye close to the ocular, and through it spot distant objects enlarged through the combination of lenses and/or mirrors.

Actually I do (and why I got modded troll, I have no idea). When you say "telescope", I think about huge things like this [man.ac.uk] - not something someone from 400 years ago could have built. And when you say "Galileo" I think "the guy who claimed earth was not the center of the universe along with Copernicus", not "the guy who invented the telescope".

No. I know the brothers Wright invented the airplane in the 1900s. I didn't associate the word "telescope" with this [eurocosm.com] because I'm not a native English speaker. Yeah, so I don't know everything. Big deal. Get over it. I asked whether 4 centuries is a typo because I wanted to learn something, is that ok with you?

Because they're not really wild hypotheses at all. You can OBSERVE the rotation curves of galaxies and see they don't match up with the estimates of the matter content. SOMETHING is there, so your only real quibble might be with the cryptic name 'dark matter'. Likewise, SOMETHING is causing the universe to expand, as shown by observations of standard candles such as Type 1A supernovae.

These are things that can be and are published in scientific journals. Whereas the only real observable evidence for t

2.) If their deflection off course was caused by a misunderstanding of gravity, the periods of the planets would have been determined to be incorrect - and if that were the case, then New Horizons would have either missed Jupiter completely or gotten a tad too close...

The difference is that there are NOT many different studies confirming ESP happens. In fact, there are many studies arguing the contrary (particularly if you focus on studies from reputable sources). There are plenty of people who WANT ESP to be true, but I don't think there are many who have been convinced by the evidence.

One big take home point about dark matter and dark energy is that physicists didn't want them to be true! It took an enormous amount of evidence, with countless independent confirmat

Which studies? How are they confirming "it happens"? What is the "it" they are confirming?

I'm sorry, but if you want to compare the top scientists of this world with a bunch of self-deceiving charlatans and quacks, and fail to find any difference, maybe it's you that need work, and not the world.

Well, one (relatively paranoid) theory is that they don't want others getting the technology. "They" in this case is the powers-that-be. The US Government has been experimenting with Remote Viewing [wikipedia.org] for many years now. Some would say successfully so. But frankly there is no good evidence for the existence of any kind of psychic power, at least nothing that I've ever seen. If it's not a controlled scientific experiment, it's useless to science. Of course that doesn't mean that such a power exists and has not

Ok so i know this is off topic, but why are wild hypotheses like this taken so seriously when things like ESP/human mind altering random probability kind of things laughed at so widely when they actually have many different studies confirming it happens?

Heh. Well, then, just send them to the Randi foundation which still has a 1 million dollar prize for anyone who can prove anything like that. The requirements so far have been reasonable too, usually along the lines of having a scientific double-blind test. Nothing you wouldn't expect in normal science. Altering probabilities is even more straightforward, since then you just have to take a large enough sample and do some elementary statistics. So you'd think that if ESP or mind-over-matter or whatever floats your fantasy boat was that proven and working, someone would claim the prize already. But, nah, suspiciously so far what we've had were:

- bullshitters arguing about how unsound scientific testing is, and why they won't take part in it (sorry, if something is only perceived when the test subjects are told and persuaded what they should perceive, then it's probably just make-belief.)

- lame stage magician tricks

- various versions of some global conspiracy to suppress them (funny how noone suppressed them before, then. You'd think the conspiracy would then stop them from publishing books and making faked movies about it too, not just stop them from taking part in a controlled experiment.)

Etc.

Plus, Randi isn't the only one who came up empty so far. What fraudsters are quick to tell you, as if it were some proof of ESP existing, is that both the USA and the USSR were interested in it during the cold war. That much is true. Unsurprisingly, since for example transmitting a message to a submarine by a mean that's (A) not blocked by water or rock, hence receivable from any depth or hole, and (B) impossible to intercept, is any army's or navy's wet dream. What they conveniently ommit there is that both the USA and the USSR, and a few others for that matter, failed to get any results with it.

By contrast, the people with these physics hypotheses tend to actually have some verifiable/falsifiable data, and they give it to you up front. If they did just bullshitting and handwaving like the ESP gang, we wouldn't take them seriously either.

Alternately, check out the Archive of Scientists' Transcendent Experiences [issc-taste.org]. The failure to "prove" ESP may show the limits of current experimental design rather than the limits of what's real. There are many, many accounts - not just in that Archive - of experiences which are inconsistent with a world in which something like ESP isn't existent. See in particular noted psychologist Charles Tart's account [issc-taste.org], and that of Susan Blackmore [issc-taste.org] - who has written one of our current textbooks on consciousness and the brai

There are many, many accounts - not just in that Archive - of experiences which are inconsistent with a world in which something like ESP isn't existent.

Data is not the plural of anecdote. Show some empirical evidence for ESP in a reproducible experiment and scientists will believe it. Until then it will be rightfully treated as the work of babbling idiots.

Ah, the delusional people, then. Yes, I forgot to mention those. They're the ones who actually end up applying for the Randi foundation prize, while the true fraudsters just argue about why they won't bother.Let's make one thing clear, since you mention psychology and the brain. In a sense you don't live in the outside world, not even in the world of your senses, but in the representation of it that reaches your upper consciousness level. When you see a car on the street, what your conscious brain sees isn'

The best way to determine the matter content of the universe is through observations of the cosmic microwave background (CMB). The properties of the plasma that emitted the CMB are well known and be used to predict temperature anisotropies (variations) in it. These show up as peaks and troughs at different angular scales. We know approximately how far away the CMB is in terms of redshift (z ~ 1100... really far!), so these angular measurements give us a distance scale. In a curved universe, the peaks and troughs appear at different angles, whereas those observed are consistent with a flat universe. A flat universe MUST have a certain energy density, but the observed baryon density only accounts for about 4% of that.

This could all be accounted for by dark matter save for the observations of Type 1A supernovae which indicate accelerating expansion, and this requires domination by a state of matter with negative pressure, and this is what's been coined dark energy.

MOND predicts the break down of GR at galaxy scales. If GR does not work at the galactic scale then it is meaningless apply it at larger scales. DM may still survive, because MOND needs it at the scale of cluster of galaxies, but that doesn't mean that we will need Dark Energy. It may be that as Gravity becomes stronger at lower accelerations, it may become repulsive at cosmic scales. This is a preliminary result of Conformal Gravity. All other forces are explained quantified Conformal Field Theories, and i

But he was using it in the opposite direction. The parent asked,"How do we know that dark matter isn't just blah blah blah", and the AC replied,"Because we can calculate blah blah blah", and I pointed out that perhaps the calculations were wrong.I'm not saying that dark matter is or isn't blah blah blah. I'm just saying that relying on calculations to assert that dark matter is or isn't blah blah blah is the wrong approach.

Nobody is saying you can't own a gun, nobody is saying you can't carry a gun... We'

To answer your first question "How do they know that the matter is not accounted for?", here's how we do it:

Step 1. Pick a galaxy
Step 2. Determine its distance using variable stars (stars that change in brightness in well-known methods).
Step 3. Determine the absolute magnitude of the galaxy (how bright it would appear if it was a fixed distance away).
Step 4. Determine how much total mass all the stars in that galaxy have in order to provide that brightness.
Step 5. Observe the doppler shift in the light from the edges of the galaxy (the side rotating toward you will appear bluer than normal, while the side rotating away from you will appear redder that normal) to determine the rotational speed of the galaxy.
Step 6. Determine how much mass must be in the galaxy in order to provide the necessary centripetal acceleration to create the observed rotational speed.
Step 7. Compare answers from Step 4 and Step 6.
Step 8. Smack yourself in the head when you realize the stars in the galaxy only account for less than 1% of the mass required to hold the galaxy together.

That is all well and good but it may be that the need for "dark energy" and "dark matter" may be the result of sloppy science. If scientists cannot tell the difference between a distant giant galaxy and a nearby dwarf galaxy, how can you believe a word they say about missing mass?

That is all well and good but it may be that the need for "dark energy" and "dark matter" may be the result of sloppy science. If scientists cannot tell the difference between a distant giant galaxy and a nearby dwarf galaxy, how can you believe a word they say about missing mass?

Doesn't this article quite clearly show that people can tell the difference, however for this particular galaxy (presumably unremarkable and not very well observed, given that apparently nobody has taken a measurement of the red-shift for the past two decades) someone messed up and they were treated using incorrect data?

Mistakes happen in all fields, to say that one particular example (or, indeed, given the human capacity to screw up, numerous ones) renders a field meaningless is highly dubious. What is

Step 6. Determine how much mass must be in the galaxy in order to provide the necessary centripetal acceleration to create the observed rotational speed using MOND, instead of GR.Step 7. Compare answers from Step 4 and Step 6.Step 8. Smack yourself in the head when you realize that there is no mass discrepancy. Then realize that GR is providing wrong answers to the questions posed by the Galaxies.MOND is ignored by most of the scientific community although it has been giving good results since 25+ years. It

According to the math and our UNTESTED theories of how black holes behave, we THINK they probably go away due to Hawking radiation. Now before someone attempts to do this in the vicinity of our solar system, I think it should be put up for a little public debate on whether or not it's absolutely necessary.

I suggest we wait to hear from an alien civilization who has tried it first.

Evolution says nothing of the sort, unless you presume abiogenesis can occur in dark matter as well as in traditional matter. Given the vast difference in the behaviour of dark matter compared to 'normal' matter (that is, the matter we deal with on a day to day basis), the possibility of abiogensis in dark matter is a fairly sizable theoretical leap (if, indeed, it's not outright precluded by the nature of the interactions), and so evolution is unable to play a part.
(Of course, all the above is in referen

I remember reading a while back that scientists had done lab experiments in which they had particles of normal matter and dark matter in a vacuum. When the particles came in contact with each other they completely obliterated one another.

I may be wrong, but it sounds like you're describing antimatter, not dark matter. Antimatter is known to exist, and it's basically like normal matter, but with the protons and electrons reversed, so that the protons (positrons) orbit the atom's nucleus.